Sounds like wake turbulence, especially if your rolls were in the same direction.
Yes, while wake does descend below an aircraft, you can still certainly experience it at the same level while in trail.
Autopilot problems can do what you describe, engaged, or not. This is why emergency checklists generally include direction for electrically disabling the autopilot once it has been interrupted in a runaway situation or malfunction (usually by pulling the appropriate circuit breaker(s)).
Last winter we had a LR35 autopilot that kept engaging itself and taking the controls in the roll mode with no pilot input. If you have a known autopilot problem, completely disable it, and certainly don't go flying and using the autopilot.